1. Field of the Invention
The present invention relates to a liquid crystal display for suppressing a change in a gap by utilizing a columnar spacer.
2. Description of the Background Art
A liquid crystal display includes a liquid crystal display panel and a driving circuit. The driving circuit supplies a predetermined signal or voltage to the liquid crystal display panel. The liquid crystal display panel generally has a pair of substrates and a liquid crystal layer disposed between the pair of substrates. The pair of substrates has an active matrix substrate and a color filter substrate, for example. Moreover, the pair of substrates is disposed to be opposed to each other. In addition, a liquid crystal display of a transmission type or a semitransmission type has an illuminating device such as a back light.
A thickness of the liquid crystal layer of the liquid crystal display panel is defined by a member which is referred to as a spacer. For the spacer, there is used a columnar spacer formed by using a photosensitive resin in a substrate.
In order to obtain high display quality, it is important to set and maintain the thickness of the liquid crystal layer to have a predetermined value. For this reason, optimization of a structure and arrangement of the columnar spacer is an important technical problem.
First of all, a characteristic for easily carrying out deformation with respect to a very small load is required for the columnar spacer. The reason is as follows. For example, in the case in which the liquid crystal display is put in a low temperature, all members constituting the liquid crystal display try to contract. In this case, a contraction rate of a liquid crystal material in the members constituting the liquid crystal display is the highest. For this reason, the gap contracts in such a direction that a gap between transparent substrates is reduced.
At this time, if the deformation of the columnar spacer cannot follow the reduction in the gap, a pressure in a liquid crystal cell of the liquid crystal display is lowered. As a result, vacuum air bubbles (low temperature foaming) in a liquid crystal cell tends to be generated.
To the contrary, in the case in which the liquid crystal display is put in a high temperature, all of the members constituting the liquid crystal display tend to expand. Also in this case, the liquid crystal material has the highest expansion rate in the members constituting the liquid crystal display. For this reason, the pressure in the liquid crystal cell is raised.
Referring to the columnar spacer, the columnar spacer is usually provided with being compressed in advance when the liquid crystal display panel is to be fabricated in order to also follow a change in the gap in an extending direction. Within a range in a certain extent, therefore, the change in the gap caused by thermal expansion of a liquid crystal can be absorbed by extension through elastic deformation of the columnar spacer.
In the same manner as in the case in which the liquid crystal display is put in a low temperature, however, if the change in the gap is excessively great, the deformation of the columnar spacer cannot follow the enlargement of the gap so that a clearance is formed between the columnar spacer and one of the transparent substrates. In other words, a gap between the transparent substrates is not held by the columnar spacer. As a result, a liquid crystal corresponding to a volume which is not held by the columnar spacer flows downward by a dead weight so that the liquid crystal is accumulated on a lower part in the liquid crystal cell. Consequently, a gap on a lower side of the liquid crystal display panel is enlarged locally so that an unevenness on a display (a gravity irregularity) is caused.
From the foregoing, it is desirable that the columnar spacer should have such a characteristic as to follow the change in the gap between the transparent substrates within a range which is as wide as possible. More specifically, it is desirable that the columnar spacer should be easily compressed and deformed against a very small load with respect to a change in such a direction that a predetermined gap is reduced, and furthermore, the columnar spacer compressed in the predetermined gap is greatly extended and deformed with respect to a change in such a direction that the gap is enlarged.
On the other hand, the columnar spacer is required to have a plastic deformation amount reduced after strong force is applied and then removed. The reason is as follows. In the case in which comparatively great weighting is locally applied to the liquid crystal display panel so that the deformation amount of the columnar spacer exceeds an elastic deformation range, a gap in a weighting application portion is maintained to be crushed and narrowed. In the case in which the plastic deformation amount, that is, a disposition after the removal of the force is large, a gap irregularity occurs.
As the case in which the comparatively great weighting is locally applied to the liquid crystal display panel, a pressure test such as a finger push test in manufacture or the like is taken as an example. Moreover, in the case of a liquid crystal display panel including a touch panel, it is also assumed that pushing is unnecessarily carried out in use and great weighting is consequently applied. In any case, the weighting is often applied to a central part of a video display region in the liquid crystal display panel. Furthermore, as compared with a peripheral edge part of the display region to which a seal for bonding transparent substrates together to seal a liquid crystal or the like is close, the weighing directly acts on the columnar spacer so that the columnar spacer is deformed in the central part.
Moreover, by a method of dropping a liquid crystal material onto substrate surfaces and then bonding substrates together (which will be hereinafter referred to as an instillation method) that is recently employed widely, a thickness of a liquid crystal layer (a cell gap) is determined depending on a drip of the liquid crystal material. For this reason, there is a problem in that a gap irregularity tends to occur when a balance is lost between the height of the columnar spacer and the drip of the liquid crystal.
As their countermeasure, Japanese Patent Application Laid-Open No. 2003-084289 discloses the technique having the structure in which two types of columnar spacers having different heights are provided and an external load within an elastic deformation range of the higher one of the columnar spacers is applied to the lower columnar spacer (which will be hereinafter referred to as the related art A). According to the related art A, it is possible to absorb the contraction of the cell gap with a change in a temperature by only the higher spacer, thereby reducing an occurrence of low temperature foaming/gravity irregularity. Moreover, when a pressure is applied, it is possible to suppress the occurrence of the gap irregularity by the action of the lower spacer.
However, the related art A has the following problem. In the related art A, it is possible to suppress a change in a gap at a high temperature or a low temperature. However, with the structure according to the related art A, a strength is not generally sufficient in the central part of the video display region to which a pressure is applied more easily than the peripheral edge part of the display region of the image.